The Coral-lovers

The free-living (but, from the look of it, not particularly mobile) coralliophiline Latiaxis mawae, photographed by Merlin Charon.

While the subjects of today's posts, the gastropods of the Coralliophilinae, might have a name that translates as 'lovers of coral', it seems likely that the corals do not love them. Members of the Coralliophilinae are a group of about 250 species of specialised predators/parasites of corals and other cnidarians. They vary in habits from free-living grazers to species that live embedded within their coral host, and as a result they vary in external appearance. However, their internal anatomy is fairly consistent, with all species lacking a radula and having a similar gut anatomy. As far as is known, they are all protandrous hermaphrodites, maturing from males to females as they grow larger. The females brood their eggs within the mantle cavity. Phylogenetic analysis has indicated a relationship with the subfamilies Rapaninae (the oyster drills) and Ergalataxinae in the Muricidae, and the recent tendency has been to treat the coralliophilines as a subfamily of the latter (Barco et al. 2010).

The endobiotic Magilus striatus, with a close-up of the coiled juvenile shell, photographed by Femorale.

Like other muricids, many of the free-living coralliophilines (such as species in the genera Latiaxis and Babelomurex) are strikingly ornamented with spiny shells. Those that live in closer association with their coral hosts, however, show a corresponding reduction in ornamentation. Members of the genus Rapa, feeding on soft corals, have inflated, relatively flat-spired shells. Most derived of all, perhaps, is the boring genus Magilus, in which the shell, after an initial coiled section, becomes uncoiled and tubular to form a channel to the outer wall of the host coral. The simplified shell morphology of the endoparasitic forms means that species can be exceedingly difficult to distinguish. A study of the genus Leptoconchus, internal parasites of mushroom corals, by Gittenberger & Gittenberger (2011) identified fourteen molecularly distinct species, each associated with a different species of coral. However, morphological variation between specimens did not reliably correlate with molecular and host-association data and could not be used to distinguish species. In comparison, a study of the host-nonspecific free-living species Coralliophila meyendorffii found that, despite the clear distinction of a larger form feeding on sea anemones vs a smaller form feeding on corals, the two forms were not resolved as molecularly distinct and there was inadequate support for the recognition of the forms as separate species (Oliverio & Mariottini 2001).

The bubble turnip Rapa rapa, photographed by Eddie Hardy.

REFERENCES

Barco, A., M. Claremont, D. G. Reid, R. Houart, P. Bouchet, S. T. Williams, C. Cruaud, A. Couloux & M. Oliverio. 2010. A molecular phylogenetic framework for the Muricidae, a diverse family of carnivorous gastropods. Molecular Phylogenetics and Evolution 56: 1025-1039.

Gittenberger, A., & E. Gittenberger. 2011. Cryptic, adaptive radiation of endoparasitic snails: sibling species of Leptoconchus (Gastropoda: Coralliophilidae) in corals. Organisms Diversity & Evolution 11: 21-41.

Oliverio, M., & P. Mariottini. 2001. Contrasting morphological and molecular variation in Coralliophila meyendorffii (Muricidae, Coralliophilinae). Journal of Molluscan Studies 67: 243-246.

(Belated) Taxon of the Week: The Bishop's Mitra

Mitra cardinalis. Photo from Sydney Shell Club.

The marine gastropods of the genus Mitra get their genus name (as well as their common name of 'mitre shells') from the resemblance of many species, at certain angles, to the pointy hat of a bishop (and indeed, the species names Mitra episcopalis, M. pontificalis and M. papalis all appear to be floating around out there). They are fairly middling-sized shells - three or four centimetres long would seem to be a respectable Mitra size - and most of them are slender and pointed at one end (the technical term is 'fusiform', and the discription 'cigar-shaped' gets bandied about regularly). Members of the subgenus Strigatella, however, are shorter, more globular animals.

Mitra are members of the family Mitridae, which is in term a family of the Neogastropoda. Neogastropods have been featured at this site before (here and here), albeit without having been identified as such, and there is a fair probability that if you go looking for gastropods on a trip to the beach that the first one you find will be a neogastropod. This is not so much because neogastropods are that much more abundant than other marine gastropods (although they are a fairly speciose bunch) as because neogastropods tend to be a lot more active than other gastropods, and are much more likely to be visibly on the move while other gastropods are sitting clamped to rocks. And the reason for the greater mobility of neogastropods is a matter of diet.


Mitra idae. Note the siphon protruding in front of the shell. Photo from Wikipedia.

The ancestral diet for gastropods was a reliable, if somewhat unexciting, scraped meal - algae rasped off rocks, or the fruits of scavenging. As a result, mobility is not at much of a premium for most gastropods - it doesn't take much speed to chase down a patch of algae - and the only reason to move is to get to the next patch of algae. Neogastropods, however, tired of this diet and went for something a little more exiting - they became active predators. Mobile neogastropods at the beach are on the hunt for prey (or, alternatively, pre-deceased animals to scavenge off). One of the most distinctive features of neogastropods to the casual observer is their elongate siphon, which in live animals can usually be seen extended from the front of the shell (which has a distinct notch or anterior extension for it to extend through), waving back and forth as the animal moves, sniffing for any appetising scents. The radula (the tongue-like structure covered with teeth in the mouth of a gastropod) has become adapted to the predatory life-style, with the number of teeth reduced but each individual tooth much larger and sharper. The fusiform body-shape as seen in most Mitra also appears well-suited to mobility, and is shared by a significant proportion of neogastropods.


Mitra mitra everting its proboscis. Photo from here, where they've made the mistake of thinking this individual is swallowing a worm.

Members of the family Mitridae possess a particularly elongate proboscis, often longer than the rest of the animal. Running along the inside of the proboscis is the radula and a muscular rod called the epiproboscis, which can be even further extended. Mitrids are specialist feeders on sipunculan worms, which live buried in sediment or burrowed into corals (Taylor, 1989), and the epiproboscis is used to capture their prey. Suggestions that it is used to inject digestive enzymes into the prey for external digestion are incorrect, as the prey is usually swallowed directly without allowing time for digestion (Taylor, 1989) The method used by Mitra idae to capture a sipunculan was described by West (1990), and as the morphology of the epiproboscis is fairly constant within the Mitridae other species probably use the same or a very similar method. After locating a sipunculan with its siphon, the gastropod would extend its proboscis until it contacted the worm, then the epiproboscis to grab onto the worm. The first move of the Mitra would then be to try and suck the worm directly out of its burrow. If this failed (which I suspect would be the norm), it would then use its radula to rasp a hole through the worm's skin before inserting the epiproboscis through the hole. The epiproboscis would entwine itself around the worm's viscera and grab directly onto its intestines. The viscera would then be hauled out through the hole in the sipunculan's skin and slurped down the waiting proboscis. Once the Mitra had pulled as much of the worm's guts out as it could, it would close its proboscis over the remaining husk and finish drawing the worm out from its hole. Insertion and retraction of the epiproboscis took a little under ten seconds. The whole process, from initial insertion to final withdrawal, could take up to twenty minutes.

REFERENCES

Taylor, J. D. 1989. The diet of coral-reef Mitridae (Gastropoda) from Guam; with a review of other species of the family. Journal of Natural History 23: 261-278.

West, T. L. 1990. Feeding behavior and functional morphology of the epiproboscis of Mitra idae (Mollusca: Gastropoda; Mitridae). Bulletin of Marine Science 46 93): 761-779.